As is known, soldering connections that realize the electrical contact-connection of the semiconductor chips are exposed to particular mechanical stress input, particularly in the event of thermal cycling. If, moreover, different materials are also joined together by means of the soldering connection, a normal and shear loading acts on the soldering connections on account of the different coefficients of thermal expansion of the materials particularly on the part of a chip, which loading may lead to the connection tearing away. In this case, the tearing-away may take place both between the soldering pad and solder and between the soldering pad and substrate.
The soldering connections of ball grid arrays that are arranged in a grid in matrix-like fashion are exposed to particular mechanical stress action on account of the entire areal extent. This is critical principally for asymmetrical ball arrangements, because local stress maxima result for individual soldering connections. As early as during the soldering process, a bulging of the substrates and components may occur, which may lead to a deformation of the balls in the z direction and also to latent stress states within the balls, thereby significantly impairing the reliability of individual soldering connections in the case where such local stress maxima occur simultaneously.
One possibility for configuring the soldering connection such that it is more resistant to stress is to use larger amounts of solder, which is achieved by enlarging the diameter of the solder balls or by increased application of solder paste. However, the increased amount of solder enhances the risk of the formation of short-circuiting bridges due to the solder repeatedly flowing out during the soldering process, since the required electrical spacings cannot be complied with, which may lead to electrical flashovers, or the solder ball electrically connects adjacent soldering pads to one another or to adjacent lines. This effect occurs particularly in the case of a spatially demanding or asymmetrical arrangement of the soldering connections.
However, the properties of soldering resist, which is applied areally on most substrates or printed circuit boards after metallization and prior to production of the soldering connections, in order to prevent solder that flows out during soldering from forming electrically conductive bridges to adjacent line structures or to adjacent soldering pads and in order to improve the electrical properties of the printed circuit boards or substrates, in particular to increase the flashover resistance, likewise influence the reliability of the soldering connection.
The soldering resist is a polymer that is not wetted by the solder during the soldering operation. Application is usually effected by means of screen printing technology or phototechnology, screen printing being used less and less often because the fit tolerance that can be achieved with it is insufficient for many applications. In the course of the application of soldering resist, the soldering pads and likewise holes and slots remain free of resist or are uncovered again by suitable methods so that the soldering resist patterned in this way forms a mask.
In terms of their configuration, the resist-free regions virtually correspond to those of the soldering pads. They are generally larger than the soldering pads and arranged centrically with respect thereto. If the fit tolerance of the soldering resist mask is insufficient, the possible non-centric arrangement of soldering pad and resist-free region means that there is the risk of soldering bridge formation between the soldering pad and adjacent the conductor track, since the minimum electrical spacing between the soldering pad and the conductor track is not complied with or the solder even covers a part of the conductor track. If a part of the soldering pad is covered by soldering resist on account of a non-centric arrangement or insufficient fit tolerance, a thin, transparent veil may arise on the soldering pad, the veil leading to soldering defects during soldering.
This last is often combated in practice by making the resist-free regions significantly larger than would be necessary in accordance with the soldering pad size. This leads not only to the possible formation of short-circuiting bridges but also to the reduction in size of the webs remaining between the resist-free regions and thus of the adhesion area present for the adhesive strength of the soldering resist mask on the printed circuit board or the substrate. Consequently, during the subsequent production process, excessively narrow webs of the soldering resist mask can detach from the substrate and cause soldering bridges.
Such soldering pads with a soldering resist mask drawn back relatively far have a further soldering location defect which is typical of ball grid arrays and the cause of which resides in cracking at the interface of the substrate in the region below the soldering pad. Under the mechanical loading described, the cracking practically leads to the peeling-away of the soldering pads including a superficial layer of the substrate and to the destruction of the line structure into which the soldering pad is integrated.
The drawing-back of the soldering resist mask is additionally limited by the grid dimension of the soldering connections of the ball grid array and by the density of the line structures on the printed circuit board, since it is necessary to comply with the required electrical spacing for ensuring the flashover resistance and the resist-free regions around a soldering pad, for the purpose of avoiding solder bridge formation, must not uncover an adjacent conductor track either.
Therefore, if in the case of relatively dense ball grids and relatively dense line structures, the open resist-free regions of the soldering resist mask are made smaller than the soldering pads situated underneath, it is necessary to develop precise soldering resist edges since even a slight coverage of the soldering pad with soldering resist has the effect that a veil that forms covers the entire area and causes soldering defects. Moreover, it has been ascertained that the peripheral sharp edge of the soldering resist exercises a notch effect on the soldering connection, thus resulting in a weakening of the connection and, even in the case of relatively low normal and shear stresses, the interruption of the soldering connection. U.S. Pat. No. 6,228,466 B1 illustrates such soldering pads with peripheral coverage of the soldering pad edge by a resist mask for ball grid array contacts.
The '466 patent likewise describes the embodiment of a contact of a wiring on a substrate, in which a resist mask covers the wiring, excluding those regions which serve for producing the soldering contact, the resist mask being drawn back completely from the surface of the contact region of the wiring apart from individual small edge regions. In order to improve the adhesion of the contact regions on the substrate surface, however, the resist mask is drawn back only to an extent such that there is a connection between the resist mask and the peripheral side areas of the contact region over the entire periphery. The resist surface is sunk below the surface of the contact region only in individual sections of the periphery and this fraction of the side area is thus uncovered for participating in the soldering connection.
What is disadvantageous in this case, however, is that the production of this particular topography of the resist mask in the vicinity of the contact region requires particularly complex and cost-intensive methods and, depending on the method, possibly also additional transport and positioning sequences. What is more, the improvement in the adhesion that can be achieved therewith is not sufficient for soldering connections with stress loadings such as occur in particular in the integration of BGA packages.
An embodiment of contact pads for soldering connections arranged in a grid-like manner is described in Japanese Patent Specification 2001230513 A, in which the areas of the pad and of the corresponding mask opening are displaced relative to one another so that a part of the pad is covered by the mask and, consequently, this mask region projects into the soldering ball and at the same time uncovers a part of the side area of the pad, as a result of which the part is included in the soldering connection. In order to achieve a sufficient coverage that withstands the tensile and shear loading to a sufficient extent, the soldering pads are always made larger than absolutely necessary for the connection. What is disadvantageous in this case, however, is that an enlargement of the soldering pads, on account of the ever-decreasing grid dimensions and the ever more demanding grid geometries, is appropriate either only in the edge region of the grid or in isolated fashion.